For material processing lasers, and in particular lasers used in ophthalmological surgery, it is essential to determine the depth of the focus of the laser beam exactly, so as to achieve a high-quality and precise cut.
Refractive lasers are a particular kind of material processing laser which are used in LASIK (laser assisted in-situ keratomileusis) surgery. LASIK surgery is performed in three steps. A first step is to create a flap of corneal tissue. A second step is reshaping or remodelling of the cornea underneath the flap with the refractive laser. In a final step, the flap is repositioned.
The human cornea consists of five layers. The outer layer is the epithelium, a thin tissue layer of fast-growing and easily regenerated cells, typically composed of about six layers of cells. Next, the Bowman's layer, which is 8-14 μm thick, is a condensed layer of collagen that protects the stroma. The stroma is a transparent middle layer, consisting of regularly arranged collagen fibers, along with sparsely distributed interconnected keratocytes, which are cells responsible for general repair and maintenance. The Descemet's membrane is a thin acellular layer around 5-20 μm thick. Finally, the endothelium is a layer, approximately 5 μm thick, of mitochondria-rich cells.
The stroma is the thickest layer of the cornea, accounting for up to 90% of the corneal thickness. A remodeling or reshaping of the stroma during surgery alters the light-focussing capability of the cornea, which results in a correction to the patient's vision.
During LASIK surgery, to control the depth of the focus of the laser beam more exactly, a flat or curved transparent or/and translucent plane is placed in contact with the outer surface of the eye. This plane is also called an applanation element. The applanation element has a front surface which is typically coated with a reflex-minimizing layer, and a back surface which is in contact with the eye.
When cutting the flap, the depth of the focus of the laser must be controlled very precisely. The flap is cut to a depth of approximately 80 μm to 500 μm, such as approximately 120 μm. The flap is typically created very near the Bowman's layer to avoid trauma caused by pulling back the flap but sufficiently far from the Bowman's layer to avoid breaching the layer. To enable consistent, high-quality results, the focus depth of the laser beam must be controllable to within a precision of a few micrometers.
In current LASIK surgery systems, the depth of the focus of the laser, relative to the cornea surface of the eye, is calibrated (or recalibrated) before the start of each surgery.
To determine the exact depth of the focus of the laser beam during calibration, the back surface of an applanation element is in contact with the eye, and a laser beam having a particular pattern is directed at the eye. The exact depth of the focus of the laser beam relative to the back surface of the applanation element is calculated based on the measured pattern of light reflected from the back surface of the applanation element. To ensure that there are no spurious reflections from other surfaces which would compromise the quality of the calculations, the front surface of the applanation element is coated with a high-transmissivity reflection-minimizing coating.